In the optical field, the sensing technologies used in precision measurements of linear displacement mainly include grating and laser interferometer.
A grating measures a displacement by way of counting the number of bands in a moiré fringe. It employs the technology of precision machinery subdivision to add densely distributed and equally spaced grating lines on the surfaces of index grating and scale grating. When in use, the index grating and scale grating are stacked together with leaving only a suitable small space therebetween; in addition, a very small angle is held between the grating lines of the two gratings. That is to say, the grating lines of the two gratings intersect with each other. Subsequently, a light source with constant intensity is used to illuminate the two gratings in a direction that is vertical to the plane defined by the intersected grating lines of the two gratings. Accordingly, light passes through the small spaces formed by the intersected grating lines of the two gratings and then generates the fringes formed by a series of light and shade strips (in other words, moiré fringe). In this context, a grating measurement for spatial displacement could be achieved by counting the number of changes in light and shade of the generated moiré fringe. The measurement accuracy and resolution of this approach depend on the fineness of grating lines. Nevertheless, it is known that the grating line fineness alone is very difficult to achieve a precision measurement of linear displacement within the range of from 1 μm to 0.1 μm. In order to achieve the foregoing measurement, a high precision electronic subdivision technology needs to be adopted. While such technology makes grating manufacturing process more difficult. In addition, the gratings produced in such way are associated with high cost and low interference capacity.
A laser interferometer functions based on the light wave interference effect, which measures a displacement by way of counting the number of wavelengths of a light wave. It is able to make a superposition of two light signals with different optical paths or different frequencies, which then generates a light intensity signal whose amplitude or phase would change along with a displacement. In this way, it is able to achieve highly precise measurements for displacements within the range of respective light wavelength. However, it is noted that this approach has a very high requirement for its laser. In addition, it is very expensive and certain external environmental factors can easily interfere with it. Moreover, it cannot be used for making measurements at working sites for a long period of time.
In recent years, a new type of time grating sensor has been developed in China. With this type, the time grating sensor uses a clock pulse as the measurement standard. The currently available time grating linear displacement sensors may be divided into two major types, namely magnetic field type and electric field type. A magnetic field type time grating makes measurements by virtue of the electromagnetic induction mechanism. However, most magnetic conducting materials have poor machinability and their production process needs winding coils. As a result, this type of time grating sensor has low production efficiency. In addition, since driving its stator coils requires a quite high driving power, its power consumption is considerably high. An electric field type time grating makes measurements by virtue of the alternating electric field coupling mechanism. For example, in the Chinese patent CN102288100B with the title “Time grating linear displacement sensor based on alternating electric field”, it has disclosed an electric field type time grating sensor. It employs the semiconductor processing technology, and thus has high production accuracy and consistency. However, due to the reasons that an electric field signal is in a divergent state in an air gap and that it requires the parallel mounting distance between its sizing body and probe body to be as short as possible, the production and installation of this sensor are relatively difficult.